I know that TA does not compute interaction, just simply because of
the way it works.
TA takes the free space pattern of the antenna as input. So any
possible cross-coupling between elements is only approximated, if at
all. Nor is it able to take into account unequal feed impedances of
the two yagis by having the lower too close to the ground, etc. An
aspect often missed is that the stacking harness has to be added to
the model with transmission lines to get this last effect right, using
a single source. Using two EZNEC sources in effect assumes you are
using two quarter wave sections and are thereby forcing equal sources
on the yagis. This frequently is not possible if the upper is rotated,
and is miserably complex if the antennas are stacked tribanders, ala
C31XR, etc.
I just let TA tell me the effect of terrain on pattern vs. flat
ground. That gives me a ratio between two TA runs at a given takeoff
angle(s). Actual gain estimation depends on next modeling the stack
with EZNEC at the heights picked in the TA runs, as one antenna, which
is assuming flat ground. I can then incorporate the ratio from the TA
run to get an answer. This only works for the single (or set of)
angle(s) that I have noted, and is a paper process. Do not know of any
way to feed the data directly across.
Practically speaking, TA gives a general guidance about antenna
placement relative to terrain that doesn't vary, e.g., at what height
does an antenna start to "peek over" the top of the "next ridge over",
or how things will do at a house you are thinking of buying, or is
there any low angle takeoff from my place to Japan, etc. Trying to get
the last 0.2 db gain out of a stack is doomed for all the reasons
enumerated by prior posts.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Tuesday, January 08, 2002 8:03 AM
Subject: Re: [TowerTalk] stacking distances -- YT and TA
At 01:23 AM 1/8/02 -0500, Tom Rauch wrote:
>
>> You have to be careful with YT, using stacks with close spacing.
>> If you plot a stack of two antennas even one foot apart,
>> YT will still show a 3 dB gain over a single antenna.
>
>Very interesting. It is amazing what is in those manuals
>that I, like most people, never bother reading.
>
>Anyway, that's why I like the new Eznec with it's average gain! You
>can look to see if average gain exceeds 0dB as an automatic red
>flag.
I think Dave's statement could be misread to mean that YT adds 3 dB
gain
for a stack over a single antenna, regardless of spacing. It doesn't.
Instead, there is a gain error that gets more obvious (larger and less
consistent with NEC-2 models) as stack spacing is brought closer.
What the YT manual says is "The internal Yagi model in YT is simple
and
does not compute interactions between individual Yagis in a stack --
YT
assumes that each antenna is a point source. For antennas stacked more
than
about a half wavelength apart this is not a problem. For example, you
should be cautious specifying spacings less than about 20 feet on 20
meters
(and proportionately scaled on other bands) because of mutual-coupling
effects between real antennas."
I verified this just now with the classic 2-high 4-element 20m yagi
stack.
At 50 and 100 ft, the stack's gain is given as 16.2 dBi, with the
stack
2-2.2 dB better at peak than either individual antenna. However, with
the
stack at 50 and 60 feet, the peak gain rises to 17.0 dBi, with the
individual antennas 2.8 to 3.2 dB below that. At intermediate
spacings
from 20 to 40 feet, the computed gain of the stack is consistently
above
that at 50 foot spacing. This looks nothing like the curve of gain
versus
spacing produced by NEC-2 modeling.
In the documentation, Dean recommends that YT be used for what it is
good
for, which is assessing the pattern effects of irregular terrain, and
that
"I would trust the results within plus/minus 3 dB."
Does anyone know whether K6STI's TA, which uses NEC antenna models,
does a
more accurate job of predicting stack gain over terrain?
73, Pete N4ZR
The revised World Contest
Station Database
is online at www.pvrc.org
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